1. Field of the Invention
The present invention relates to a process for manufacturing a semiconductor device having a crystalline semiconductor. The present invention further relates to an electro-optical device such as an active matrix liquid crystal device using the semiconductor device.
2. Prior Art
Thin film transistors (referred to simply hereinafter as “TFTs”) are well known and are widely used in various types of integrated circuits or an electro-optical device, and particularly used for switching elements provided to each of pixels of an active matrix(-addressed) liquid crystal display device as well as in driver elements of the peripheral circuits thereof.
An amorphous silicon film can be utilized most readily as the thin film semiconductor for a TFT. However, the electric characteristics of the amorphous silicon film are disadvantageously poor. The use of a thin film of polysilicon (polycrystalline silicon), which is a crystalline silicon, can solve the problem. Crystalline silicon is denoted as, for example, polycrystalline silicon, polysilicon, and microcrystalline silicon. The crystalline silicon film can be prepared by first forming an amorphous silicon film, and then heat treating the resulting film for crystallization.
The heat treatment for the crystallization of the amorphous silicon film requires heating the film at a temperature of 600° C. or higher for a duration of 10 hours or longer. Such a heat treatment is detrimental for a glass substrate. For instance, a Corning 7059 glass commonly used for the substrate of active matrix liquid crystal display devices has a glass distortion point of 593° C., and is therefore not suitable for large area substrates that are subjected to heating at a temperature of 600° C. or higher.
According to the study of the present inventors, it was found that the crystallization of an amorphous silicon film can be effected by heating the film at 550° C. for a duration of about 4 hours. This can be accomplished by disposing a trace amount of nickel or palladium, or other elements such as lead, onto the surface of the amorphous silicon film.
The elements above (hereinafter referred to as “catalyst elements capable of accelerating the crystallization of an amorphous silicon film” or simply as “catalyst elements”) can be introduced into the surface of the amorphous silicon film by depositing the elements by plasma treatment or vapor deposition, or by incorporating the elements by ion implantation. The plasma treatment more specifically comprises adding the catalyst elements into the amorphous silicon film by generating a plasma in an atmosphere such as gaseous hydrogen or nitrogen using an electrode containing catalyst elements therein in a plasma CVD apparatus of a parallel plate type or positive columnar type.
However, the presence of the catalyst elements in a large quantity in the semiconductor is not preferred, because the use of such semiconductors greatly impairs the reliability and the electric stability of the device in which the semiconductor is used.
That is, the catalyst elements are necessary in the crystallization of the amorphous silicon film, but are preferably not incorporated in the crystallized silicon. These conflicting requirements can be accomplished by selecting an element which tend to be inactive in crystalline silicon as the catalyst element, and by incorporating the catalyst element at a minimum amount possible for the crystallization of the film. Accordingly, the quantity of the catalyst element to be incorporated in the film must be controlled with high precision.
The crystallization process using nickel or the like was studied in detail. The following findings were obtained as a result:
(1) In case of incorporating nickel by plasma treatment into an amorphous silicon film, nickel is found to intrude into the film to a considerable depth of the amorphous silicon film before subjecting the film to a heat treatment;
(2) The initial nucleation occurs from the surface from which nickel is incorporated; and
(3) When a nickel layer is deposited on the amorphous silicon film, the crystallization of an amorphous silicon film occurs in the same manner as in the case of effecting plasma treatment.
In view of the foregoing, it is assumed that not all of the nickel introduced by the plasma treatment functions to promote the crystallization of silicon. That is, if a large amount of nickel is introduced, there exists an excess amount of the nickel which does not function effectively. For this reason, the inventors consider that it is a point or face at which the nickel contacts the silicon that functions to promote the crystallization of the silicon at lower temperatures. Further, it is assumed that the nickel has to be dispersed in the silicon in the form of atoms. Namely, it is assumed that nickel needs to be dispersed in the vicinity of a surface of an amorphous silicon film in the form of atoms, and the concentration of the nickel should be as small as possible but within a range which is sufficiently high to promote the low temperature crystallization.
A trace amount of nickel, i.e., a catalyst element capable of accelerating the crystallization of the amorphous silicon, can be incorporated in the vicinity of the surface of the amorphous silicon film by, for example, vapor deposition. However, vapor deposition is disadvantageous concerning the controllability of the film, and is therefore not suitable for precisely controlling the amount of the catalyst element to be incorporated in the amorphous silicon film.